For what it’s worth, I’d probably have gone with 2u as well. I’ve always been more comfortable working the dose up from below rather than being aggressive with dosing and dealing with the drama that comes with trying to bring up scary low BGs. Plus, you didn’t arrive at 4u slowly and methodically, so it’s very possible that it’s way too much, not just a little too much. Or not. Only time and data will tell. The only caveat is the DKA history, which is an important consideration. However, a negative ketone test is great news and the fact that he’s eating well is also important.
Lantus likes consistency, so I’d stick with the 2u dose you’ve chosen to see what it’ll do, even if you see some higher numbers in the short term (he might bounce from todays lows, but just ride it out and don’t be tempted to increase; chasing numbers rarely works). Of course if at any point he drops under 90 again, another dose reduction is warranted. And I’d keep checking for ketones.
Again, good job today. It’s a lot to handle so early in the game but you’re doing all the right things. It will get easier.